Organic electroluminescent (EL) devices are generally composed of three layers of organic molecules sandwiched between transparent and metallic electrodes, the three layers include an electron transporting layer, an emissive layer and a hole transporting layer.
There are several variations in organic EL structures depending on where the emissive layer is positioned. Tsutsui and coworkers proposed three EL cell structures: SH-A, SH-B and DH (T. Tsutsui, et. al, Photochem. Processes Organ. Mol. Syst., Proc. Meml. Conf. Late Professor Shigeo Tazuke, 437-50 (1991)). The SH-A cell is successively composed of a layer of Mg-Ag as a cathode, an electron transporting layer (ETL), a hole transporting layer (HTL) and a layer of Indium-Tin-oxide (ITO) as an anode, wherein the light emission comes from the part of the electron transporting layer close to the hole transporting layer. The SH-B cell is also successively composed of Mg-Ag as a cathode, an electron transporting layer, a hole transporting layer and a layer of ITO as an anode, wherein the light emission comes from the part of the hole transporting layer close to the electron transporting layer. The DH cell is successively composed of Mg-Ag as a cathode, an electron transporting layer, an emitter layer, a hole transporting layer and a layer of ITO as an anode, wherein the emitter layer is an independent layer sandwiched between the electron transporting layer and the hole transporting layer.
Earlier, in U.S. Pat. No. 4,539,507 (entitled "Organic Electroluminescent Devices Having Improved Power Conversion Efficiencies", issued Sep. 3, 1985), VanSlyke and Tang also disclosed a SH-A type of organic EL device with a hole-injecting zone and an organic luminescent zone wherein the luminescent zone comprises an electron transporting compound, and has a quantum efficiency of at least 0.05% and a w/w efficiency of at least 9.times.10.sup.-5, and a thickness of less then 1 .mu.m.
Hamada and coworker in 1995 reported a modified SH-B type of organic EL cell (Y. Hamada et. al, Jpn. J. Appl. Phys. 34 (1995), L824-L826) with Rubrene as a dopant. The device has a luminance of 1020 cd/m2 at a current density of 10 mA/cm2 and a half lifetime of 3554 hour with initial luminance of 500 cd/m2, which is a substantial improvement over any prior known SH-B type of cells.
The objective of the present invention is to provide an organic EL device of the SH-B type which has high efficiency and good reliability.